1. Field of the Invention
The present invention relates to a field emission cold cathode and a manufacturing method of the same, more particularly to a structure of the field emission cathode having an improved insulating characteristic and a manufacturing method of the same.
2. Description of the Related Art
A field radiation cold cathode has been developed as an electron source which takes the place of a hot cathode utilizing a thermoelectric emission. The field-emission cold cathode generates a high electric field of more than 2 to 5.times.10.sup.7 cm V/cm at the tip of its electrode having an acute protrusion to emit electrons into a space. Therefore, a device characteristic depends on a sharpness of the tip of the electrode, and it has been said that a radius of curvature of the tip of the electrode must be less than about several hundreds of angstroms. Furthermore, to generate the electric field, the electrodes have to be disposed at a short distance of about 1 .mu.m or less from each other, and it has to be applied with a voltage of several hundreds of volts. As many as several thousands to several ten thousands of such elements, as described above, are practically formed on a single substrate, and they are often connected to each other in parallel so that they are used as arrays. Accordingly, the field-emission cold cathode is generally manufactured applying a fine processing technology.
One of the manufacturing methods of such a field-emission cold cathode is the one developed by Spindt et al. of SRI (Stanford Research Institute) and disclosed in Journal of Applied Physics 39, p. 3504, 1968. In this manufacturing method, an electrode having an acute protrusion in its tip can be obtained by depositing a refractory metal such as molybdenum on a conductive substrate. This manufacturing method is shown in FIGS. 20A to 20D. First of all, a silicon substrate 31 is prepared, and an oxide film is grown on the silicon substrate 31 to form an insulating layer 32. Subsequently, molybdenum is deposited as a gate layer 34 by means of a vacuum evaporation technique. Thereafter, a photoresist layer 36 having an opening 37 of the diameter about 1 .mu.m is formed by means of photolithography technique (FIG. 20A). The gate layer 34 and the insulating layer 32 are etched using the photoresist layer 36 as a mask (FIG. 20B). After the photoresist layer 36 is removed, an aluminum sacrifice layer 38 is formed by performing a rotary slanting evaporation technique. Subsequently, molybdenum is evaporated onto the resultant structure from a vertical direction under vacuum, thereby forming an emitter electrode (FIG. 20C). Finally, the molybdenum film 30 deposited on the sacrifice layer 38 is lifted-off by selectively etching the sacrifice layer 38, thereby obtaining a device structure (FIG. 20D).
The element manufactured as described above is supplied with a voltage in such a manner that the emitter electrode 35 is biased negatively and the gate layer 34 is biased positively. Thus, electrons are emitted from the tip of the emitter electrode 35 in the direction perpendicular to the silicon substrate 31. Such structure is generally termed a vertical field-emission cold cathode.
Some structures of the vertical field-emission cold cathode, and manufacturing methods thereof, have been known in addition to the foregoing structures.
In Japanese Patent Application Laid Open Heisie 4-167326, a technology for manufacturing a field-emission cold cathode is disclosed in which an inner side surface 39 of an insulating layer 32 is made to be a tapered shape in cross-section (FIG. 21). Such a shape can be obtained by forming an untapered cavity in the insulating layer 32 with an anisotropic etching technique and then by lightly etching the side surface of the cavity formed in the insulating layer 32 using hydrofluoric acid of 1 to 10%. Thereafter, the device structure of the field-emission cold cathode can be obtained using the same processes shown in FIGS. 20A to 20D.
In Japanese Patent Application Laid Open Heisei 4-262337, a technology for manufacturing a field-emission cold cathode, in which a visor-shaped overhang is made utilizing an ion-implantation of boron, is disclosed as shown in FIGS. 22A to 22D. The summary of manufacturing processes of the field-emission cold cathode therein is as follows. An oxide film 42 is formed on a silicon substrate 41, and a polycrystalline silicon film 43 is formed on the oxide film 42 by means of a CVD technique. After boron ions are implanted into the entire surface of the polycrystalline silicon film 43, an opening portion 46 is formed by means of a photolithography technique and an etching technique (FIG. 22A). Subsequently, a thermal oxidation is conducted to form an oxide layer 45 (FIG. 22B). The oxide layer 45 is removed utilizing the difference between the etching rate of the oxide film 44 and that of the oxide layer 45, the oxide film 44 is doped with boron by the ion implantation. In addition, a photoresist is filled in the opening portion 46, and the surface of the oxide film 44 is flattened to form an opening portion 47 having a visor-shaped overhang (FIG. 22C). Subsequently, a metal is deposited by means of a vacuum evaporation technique to form simultaneously an emitter electrode 48 and a gate layer 40. Thus, a device structure of the field-emission cold cathode can be obtained (FIG. 22D).
Electrons emitted from the foregoing field-emission cold cathode disperse at a divergence angle of approximately 30.degree.. Hence, as shown in FIG. 23, a field-emission cold cathode having the following multilayer-stacked structure has been disclosed. Specifically, an intermediate insulating layer 78 is further formed on a gate layer 74, and a control electrode layer 79 to suppress the divergence of the electron beam is formed on the intermediate insulating film 78. The summary of manufacturing processes will be described below. First, an insulating layer 72 made of an oxide film is grown on a silicon substrate 71, and a polycrystalline silicon film serving as a gate layer 74 is grown on the insulating layer 72. An oxide film serving as the intermediate insulating layer 78 is grown, and a polycrystalline silicon layer serving as the control electrode layer 79 is grown on the intermediate insulating layer 78 (FIG. 24A). Thereafter, a photoresist layer 76 is formed by a photoresist technique, and the control electrode layer 76 and the intermediate insulating layer 78 are etched anisotropically in this order whereby an opening portion 77 reaching to the surface of the gate layer 74 is formed (FIG. 24B). Subsequently, after removing the photoresist layer 76, an oxide layer is formed by the CVD technique, and then the oxide layer is subjected to an anisotropic etching performed vertically whereby the surface of the gate layer 74 is exposed. Hence, a side wall 80 is formed (FIG. 24C). Next, the gate layer 74 and the insulating layer 73 are subjected to an anisotropic etching in this order. Hence, a structure having diameters of the openings of the gate layer 74 and the control electrode layer 79 different from each other can be obtained (FIG. 24D). Finally, after an emitter electrode is formed by means of the vacuum evaporation technique, the side wall 80 is selectively etched whereby the device structure shown in FIG. 23 can be obtained.
In the field-emission cold cathode, since a voltage more than several tens of volts is applied between the electrodes disposed at intervals of as little as about 1 .mu.m as described above, an insulating characteristic between the electrodes of withstanding high voltage and having a low leakage current is one of the essential characteristics. Specifically, when the insulating element's ability to withstand voltage is low, the element is apt to be easily broken such that the field-emission cold cathode suffers a fatal damage. Moreover, when the leakage current is large, a quantity of power consumption increases and a stable operation of the element is disturbed.
Furthermore, since the field-emission cold cathode is used in the form of an array where a plurality of elements forming the device are arranged in an array fashion, if only one of the elements is broken for some reason and the broken element is short-circuited, the entire device fails to operate. Therefore, when some of the elements are broken, the broken element must be open-circuited and the break of the element must not affect other elements around the broken one.
In the section structure shown in FIG. 21 (Japanese Patent Application Laid-Open Heisei 4-167326) among the foregoing known section structures, the gate layer 34 has no overhang protruding from the insulating layer 32, and the whole of it is supported by the insulating layer 32. The section structure thereby processes a high strength. However, the section shape, i.e., the section shape of the opening surrounded by the side surface of the insulating layer 32, is tapered such that the opening is broader as it proceeds to the substrate 31. The electrons emitted-from the triple junction 39, in which the substrate 31, the insulating layer 32, and the space contact, are present continuously at an angle at which the wall of the insulating layer 32 crosses against a direction accelerated by an electric field. Therefore, there has been a problem that the insulating characteristic is deteriorated due to an electron collision against the surface of the insulating layer 32 and a secondary electron emission.
Furthermore, in the section structure shown in FIG. 22 (Japanese Patent Application Laid-Open Heisei 4-262337), the surface on which the emitter electrode 48 is formed is situated on a lower level than the surface of the substrate 41. Hence, a shape of the triple junction 49 at which the silicon substrate 41, the oxide film 42, and the space contact is almost circular concave. For this reason, an electric field is apt to be concentrated at this portion such that the insulating material's ability to withstand voltage is unfortunately reduced.
On the other hand, in the field-emission cold cathode which comprises the control electrode layer 79 as shown in FIG. 23, a voltage more than several tens of volts is applied between the gate layer 74 and the control electrode layer 70 and, therefore, the insulating characteristic between the gate layer 79 and the control electrode layer 79 is mentioned as one of the essential characteristics. Specifically, when the insulating breakdown voltage is low, the element is apt to be easily broken such that the field-emission cold cathode suffers a fatal damage. Moreover, when the leakage current is large, a quantity of power consumption increases and a stable operation of the element is disturbed.